Display system

ABSTRACT

A projecting display system includes a light source ( 101 ) which produces light which is spatially modulated by a number of spatial light modulators ( 105, 107 ). A splitting means ( 103 ) is provided in the light path between the light source ( 101 ) and the spatial light modulators ( 105  and  107 ) such that the overall luminous flux produceable by the system is not determined by the maximum luminous flux which each spatial light modultors ( 105, 107 ) can accommodate.

This invention relates to display systems. The invention has particular,although not exclusive, relevance to display systems including aprojection system in which light from a light source is modulated by aspatial light modulator device, the modulated light then being projectedonto a projection screen.

Spatial light modulator devices suitable for use in such projectionsystems may take several forms.

One example of a spatial light modulator device is a liquid crystaldevice comprising a matrix of individually addressable liquid crystalpixels. Such spatial light modulators may act either in a transmissivemode in which the light passes through the pixels of the liquid crystaldevice, or in a reflective mode in which the light is reflected by eachpixel of the liquid crystal device.

Another example of a spatial light modulator device is a deflectablemirror device (DMD) which comprises an array of mirrored cantilever beamstructures, each structure carrying an electrode so as to beelectrostatically deflectable between two positions. Thus, dependent onthe electric field applied to the device, each mirrored structure willreflect an incident light beam into two alternative light paths, eithertowards an optical system for projection onto a projection screen, oralternatively into a beam dump. Using an array of such structures, eachstructure being individually addressable by part of the incoming lightbeam, the incoming light beam can be spatially modulated with a twodimensional image which can then be projected onto the projectionscreen.

Known projection systems in which light from a light source is modulatedby a spatial light modulator device suffer the disadvantage that thereis often a limit in the amount of light flux which can be directed ontothe spatial light modulator. This limit is caused by, for examplelimitations associated with the heating effect of the radiant flux, orsaturation due to a high luminous flux. Where projection systems arelimited in light output, two or more projection systems may be “stacked”by placing the projection systems adjacent to each other such that theprojected images are superimposed on the projection screen, producing anoverall bright image. However, such an arrangement is both inefficientand space consuming.

U.S. Pat. No. 5,035,475 discloses a display system comprising twospatial light modulators in the form of an array of movable mirrors.Baffles in the form of rows of parallel slits are interposed in thelight path between the light source and the two mirror arrays. A beamsplitter is effective to split light from the light source between thetwo mirror arrays, and to recombine light reflected along the normals tothe mirror arrays. The baffles are effective to absorb light which isreflected along other directions. By use of the two mirror arrays, theimages produced by the two arrays may be interleaved to remove darkstripes in the projected images which are produced by the two baffles.However in such arrangement the flux of the light in the final projectedimage is still limited by the flux of light produced by each mirrorarray.

Problems also occur in colour projection systems comprising one or morespatial light modulator devices. In order to achieve a colour projectionsystem it is known to split the incoming light by one or more spectralsplitting devices, for example, dichroic mirrors into three primarycolour channels. An example of such a prior art system is shown in FIG.1 which is a schematic diagram of an overview of a colour projectionsystem using three spatial light modulators in the form of DMDs.

Referring to FIG. 1, the particular example of a display system to bedescribed is arranged to project a colour image onto a display screen101. The display system includes a light source 103 arranged such thatthe beam from the source is directed onto three planar deflectablemirror display devices 105,107,109. described.

Positioned in the light path between the light source 103 and the firstdeflectable mirror device 105 are two dichroic mirrors 111,113. Thefirst dichroic mirror 111 is designed and angled to reflect blue lightonto the second planar deflectable mirror display device 107 andtransmit all other incident light. The second dichroic mirror 113 isdesigned and angled so as to reflect red light onto the third planardeflectable mirror device 109 and transmit the remaining green componentof the light from the source 103 onto the first deflectable mirrordisplay device 105.

The three deflectable mirror devices 105,107,109 are arranged to becapable of reflecting the three colour components of the beam from thesource 103 so as to direct the spatially modulated beam through aprojection lens 115 onto the display screen 101.

However such arrangements do not take account of the fact that theluminous flux of the various spectral components, for example theprimary colours red, green and blue within white light, is unequal.

It is an object of the present invention to provide a display device inwhich the above problems of limited output light are at leastalleviated.

According to a first aspect of the present invention there is provided adisplay system comprising a plurality of spatial light modulators andincluding extra spatial light modulators designed to increase the totallight flux spatially modulated by the modulators.

According to a second aspect of the present invention there is provideda display system comprising: a light source; at least two spatial lightmodulators; means for splitting light of the same spectral compositionfrom the light source between the spatial light modulators; means forcombining spatially modulated light produced by the spatial lightmodulators; and means for displaying the combined light such that thecombined light is of greater light flux than the light produced by anyof the spatial light modulators.

According to a third aspect of the present invention there is provided adisplay system comprising: a multi wave length light source; a pluralityof spatial light modulators; wavelength selective means for splittinglight of different spectral composition between the spatial lightmodulators; means for combining spatially modulated light produced bythe spatial light modulators; and means for displaying the combinedlight; wherein there are provided sufficient spatial light modulators toincrease the balance of the division of the light flux produced by thelight source between the spatial light modulators.

The means for displaying suitably comprising means for projecting thecombined light onto a projection screen.

A number of embodiments of the invention will now be described by way ofexample only with reference to the accompanying figures in which:

FIG. 1 is a schematic diagram of an overview of a prior art colourprojection system as has already been described;

FIG. 2 is a schematic diagram illustrating the operation of a DMD;

FIG. 3 illustrates the illumination of a mirror device in the array ofFIG. 2;

FIG. 4 is a schematic diagram of a display system in accordance with afirst embodiment of the invention;

FIG. 5 is a schematic diagram of a display system in accordance with asecond embodiment of the invention; and

FIG. 6 is a schematic diagram of a display system in accordance with athird embodiment of the invention.

Referring firstly to FIGS. 2 and 3, each deflectable mirror device(DMD), for use in a display system in accordance with an embodiment ofthe invention comprises an array 117 of m×n deflectable mirror devices,typically 768×576 mirror devices for a low resolution display system or2048×1152 mirror devices for a high resolution display system. Eacharray 117 is connected to a driver circuit 119 which receives anelectronic colour video signal from the control circuit indicatedgenerally as 121, and addresses each of the mirror devices M_(ll)-M_(mn)as, for example, described in the applicant's earlier InternationalPatent Application, PCT/GB92/00002 dated Jan. 4, 1992 (incorporatedherein by reference).

Dependent on the applied address signal, each mirror device M is causedto take one of two different positions corresponding to an “on” state inwhich light reflected from the mirror device M is directed in a firstpath 123 and an “off” state in which the reflected light is directed ina second path 125. The second path 125 is chosen such that lightreflected along this path is directed away from the optical axis of thedisplay system and thus does not pass into the projection lens (notshown in FIGS. 2 and 3).

Thus, each DMD array 117 is capable of representing a two dimensionalimage, those mirror devices M which are tilted to the “on” stateappearing bright and those which are tilted to the “off” state appearingdark. By varying the ratio of the “on” period to “off” period, that isby a temporal modulation technique, grey scale can be achieved.

Turning now particularly to FIG. 3 the angle through which each mirrordevice M is deflected between the “on” state and the “off” state isrelatively small. Thus in order to achieve good discrimination betweenthe “on” and “off” states the incident light beam 127 from the source103 is directed towards each spatial light modulator 105,107,109 at anangle measured from the normal to each device of around 20°.

When an individual mirror device M is lying parallel to the plane of thearray 117, an incident beam 127 from, for example an arc lamp (not shownin FIG. 3) is reflected at a corresponding angle of 20° to the normalalong an “off” path 122 into a beam dump (not shown). When the controlsignal from the driver circuit 119 sets the mirror device M into a firstdeflection state at a first angle to the plane of the array 117, theincident beam 127 is reflected along the direction 125 in a further“off” path into the beam dump. When the control signal from theaddressing circuit 119 sets the mirror device M into a second deflectionstate at a second angle to the plane of the array 117, the incident beam127 is reflected out along the normal to the array along the “on” path123.

Turning now to FIG. 4, in the first embodiment of the display device inaccordance with the invention, a light source 401, for example an arclamp, is arranged to direct light onto a half silvered mirror 403. Thehalf silvered mirror 103 is effective to split the incident lightbetween two spatial light modulators 405, 407 each in the form of a DMD.Spatially modulated light from the DMDs 405, 407 is recombined at themirror 403, from which it passes through a projection lens 409 to beprojected onto a projection screen 411. The DMDs operate as described inrelation to FIGS. 2 and 3 in order to spatially modulate the incomingbeam to produce an image for projection screen 411.

It will be seen that by use of the half silvered mirror 403, the lightwhich would in prior art arrangements have been incident on a single DMDis split between the two DMDs 405, 407. Thus the luminous flux incidenton the projection screen 411 is twice that which would have beenpossible using a single DMD as in the prior art arrangements.

It will be appreciated that other forms of light splitters to a halfsilvered mirror may be used to split the incident light beam between thetwo DMDs 405,407. One possibility is to replace the half silvered mirror403 in FIG. 4 by a polarizing beam splitter. The polarizing beamsplitter will be effective to split incident light from the light source401 into P-polarized and S-polarized light. The P-polarized light isdirected to one of two DMDs 405, 407 and the S-polarized light isdirected towards the other of the DMDs 407 or 405. The polarizingsplitter 401 will then recombine the spatially modulated light from thetwo DMDs 405, 407 for transmission to the projector lens 409.

It will be appreciated that a polarized splitter is more efficient thana half silvered mirror. The optical losses produced by the insertion ofsuch a polarizing splitter will typically be 1-3% of the incident beamcompared to the 20-30% losses produced by a mirror surface. However evenif a relatively high loss beam splitter is used it is found that ahigher light flux projected image may be produced then would otherwisehave been possible.

Referring now to FIG. 5, the second embodiment of the invention to bedescribed is a multi-colour projection system. As seen in FIG. 5 themulti colour projection system includes four spatial light modulators inthe form of DMDs 501, 503, 505 and 507. Three dichroic mirrors 509, 511and 513 are arranged in the light path between the DMDs 501,503,505 and507 and a white light source (not shown). The first dichroic mirror 509in the light path is arranged to reflect red light onto the DMD 501 andtransmit all other light. The second dichroic mirror 511 is arranged toreflect blue light onto the DMD 503 and to transmit the remaining greenlight. The third dichroic mirror 513 is designed to have a closelycontrolled band-pass characteristic so as to reflect part of theincident green light with a chosen spectral content onto the third DMD505 and to transmit the remaining green light onto the fourth spatiallight modulator 507.

Each DMD 501, 503, 505 and 507 is driven by address signals as describedin relation to FIGS. 2 and 3 to provide an appropriately spatiallymodulated image in one the primary colours red and blue and the twospectral portions of green. The DMDs 501, 503, 505 and 507 are arrangedsuch that the reflected spatially modulated light is reflected back, andrecombined by the various dichroic mirrors 509, 511, 513 to produce amulti-wavelength spatially modulated light beam, which is then arrangedto pass back along the optical axis of the system through a projectionlens (not shown in FIG. 5) and to be projected onto a projection screen(not shown in FIG. 5).

It will be appreciated that where the spatial light modulators are inthe form of a matrix of mirrored surfaces, then a handedness is providedat the reflection at the mirror array which produces the spatiallymodulated light. This will be compensated for by the subsequentreflection at the appropriate reflective surfaces 509, 511 or 515. Inorder to provide a compensatory reflective surface for the return lightpath from the spatial light modulator 507 a reflector 515 is providedthe spatial light modulator 507 being positioned accordingly.

It will be appreciated that as in the first embodiment alternativesplitters to the dichroic mirror 413 may be used. A particularlyefficient way of splitting the green light is to introduce a polarizedsplitter such that separated P-polarized and S-polarized green lightwithin the same wavelength band is incident on the two spatial lightmodulators 505 and 507. Alternatively a half-silvered mirror may beused.

It will be appreciated that in the example given before, as white lightgenerally contains more green light than red or blue light, there aretwo green spatial light modulators. However in some circumstances it maybe appropriate to have some other combinations of light modulators whichare effective to share the total light flux in convenient proportionsamongst the spatial light modulators. For example, as white lighttypically comprises 30% red light, 60% green light and 10% blue light,the number of spatial light modulators sharing the total light flux ineach colour channel can be set according to the light flux in eachcolour channel. Such an arrangement can be used to improve colourfidelity as well as enhancing light output from the same light input.

It will be appreciated that whilst the second embodiment is described inrelation to the splitting of the input light into the primary coloursred, blue and green, the invention is equally applicable to thesplitting of the input light into the secondary colours yellow, magentaand cyan, or any other colour splitting scheme.

It will be seen that in the embodiment of the invention described inrelation to FIG. 5, the input light is split into three different colourcomponents such that the three components are spatially modulatedsimultaneously. However, in a display system in accordance with theinvention, light of different colours may be passed sequentially throughthe display system. Such an arrangement is shown in FIG. 6 in whichlight from a light source comprising an arc lamp 601 and a condenserlens 603 is arranged to pass sequentially through different portions ofa colour wheel 605 carrying red, blue and green filters. The wheel 605is rotatable by a motor 607 about a central axis so as to superimpose insequence, the red, blue and green filters in the light path from thelight source 601, 603.

As in the first embodiment described in relation to FIG. 4, a polarizingbeam splitter 609 is placed in the light path, the polarizing beamsplitter being effective to divide the incident light into P-polarizedand S-polarized light components. The P-polarized light is directedtowards a first spatial light modulator in the form of a DMD 611, whilstthe S-polarized light is directed towards a second spatial lightmodulator DMD 613.

Spatially modulated light from the spatial light modulators 611, 613passes back to the beam splitter 609, which re-combines the S andP-polarized light and directs it towards a projection lens 515 forprojection onto a projection screen (not shown).

The speed of rotation of the colour wheel 605 is chosen such that thetime for light of all three different colour components to pass throughthe display system and be projected on the projection screen, is shortenough such that the eyes of an observer watching the projection screenintegrate the three different coloured projected images on the displayscreen, and a full colour image is seen by the observer.

It will be appreciated that as in the other embodiments describedherebefore, whilst the use of a polarized splitter is particularlyadvantageous, other forms of splitter may be used.

It will also be appreciated that whilst the invention has particularapplication to display systems using spatial light modulators in theform of DMDs as such system are of ten limited by the flux handlingcapabilities of the DMDs, the invention is also applicable to displaysystems including other forms of spatial light modulators, for exampleliquid crystal devices.

What is claimed is:
 1. A display system comprising: a multi wavelengthlight source; a plurality of deflectable mirror devices, each devicehaving a plane defined by an array of mirror elements, each mirrorelement corresponding to a respective pixel of a color image to bedisplayed and being deflectable between a first orientation relative tosaid plane of the array effective to reflect light incident on themirror element at a predetermined angle not normal to the plane of thearray along an “ON” path for the device and a second orientationrelative to said plane of the array effective to reflect light incidenton the mirror element at said predetermined angle along an “OFF” pathfor the device; input circuitry for receiving signals representative ofa color image; control means for supplying address signals to eachdevice representative of a wavelength band component of said color imageand effective to control the orientation of each of the mirror elementsof each array between said first and second orientations dependent onsaid color image, the control means being arranged to supply the sameaddress signals corresponding to at least one of said wavelength bandcomponents to at least two of said deflectable mirror devices; awavelength selective means for splitting light from said light sourcewithin each of said different wavelength band components of said colorimage; means for further splitting the light in said one wavelength bandcomponent between said at least two deflectable mirror devices; meansfor directing the split and further split light in each wavelength bandcomponent towards the correspondingly addressed deflectable mirrordevice; and means for combining the light reflected by all of thedeflectable mirror devices along the respective “ON” paths for thedevices to form a single spatially modulated light beam.
 2. A displaysystem according to claim 1 wherein said means for further splittinglight comprises at least one polarized splitter effective to split thelight incident thereon into P-polarized light and S-polarized lightcomponents.
 3. A display system according to claim 1 wherein said meansfor further splitting the light comprises: at least one semi-reflectivemeans which is effective to reflect part of the light incident thereonand to transmit the rest of the light incident thereon.
 4. A displaysystem according to claim 1 wherein said means for further splitting thelight comprises a dichroic means effective to split light within twodifferent narrow wavelength bands within a broad wavelength bandincluding said narrow wavelength bands.
 5. A display system according toclaim 4 wherein the same wavelength band component is a primary colorwavelength band component and said dichroic means is arranged to splitlight within two different wavelength bands within said primary colorband.
 6. A display system according to claim 5 in which there areprovided at least two spatial light modulators in respect of twodifferent wavelength bands in the green light wavelength band component.